In a prior patent application (the U.S. Patent application Ser. No. 223,142 listed above), an electro-mechanical magnetic tape degausser was described, in which a transport mechanism was provided in order to automatically convey magnetic tape reels towards, between, and away from a pair of electromagnets. The device described in the above-mentioned application also included a tape height sensor which was used to prevent damaging contact between the magnetic tape reels and the electromagnets as the reels were conveyed towards the electromagnets by the transport mechanism.
Many types of devices are known in the prior art for erasing magnetic recording tapes. One example is U.S. Pat. No. 2,826,643 to Bruce Greiner, which discloses a method of erasing magnetic tapes by placing the tape between two demagnetizing coils, which are connected in parallel. That patent discloses the use of a threaded rod having a handwheel for moving the two coils towards and away from each other in order to accommodate recording tape of differing widths. That patent also discloses the use of a motor in combination with a roller for use in rotating a magnetic tape roll by contacting the roll on its outer circumference.
Another example of a prior art tape degausser is Rettinger U.S. Pat. No. 2,730,570, which shows the use of a pair of oppositely mounted "E" shaped magnetic cores for erasing a length of magnetic tape. From FIG. 2 of that patent, it appears that the sections of the magnetic cores have slightly narrowed tips.
The major purpose of bulk magnetic tape eraser or degausser machines is to effectively remove information which has been magnetically encoded onto magnetically sensitive tape which is stored on reels, cassettes, or some other form of cartridge or container. It is desirable that such tape may be erased quickly in bulk, while it is still on the reel or cartridge in which it is normally stored. For example, it is desirable that reels of magnetic video tape for television use can be quickly erased and reused to record new video signals without encountering any latent remains of previously recorded video signals. Such tape erasers are valuable for use in erasing video tapes, audio tapes, and computer tapes.
It is desirable that magnetic tape erasers be capable of erasing tapes stored on the large variety of reels, cassettes, and cannisters which are in commercial use without removing the tape from its container. It is also desirable that magnetic tape erasers be capable of adequately and fully erasing modern high-coercivity tapes made using chromium or other metals. Because of higher recent performance standards for magnetic recording tapes, it is important that magnetic tape erasers make as complete as possible an erasure of information encoded on a tape so as to prevent remnant information of prior recording from contaminating later recordings.
A problem with prior magnetic tape erasers is that, when high-strength magnetic fields are passed through the magnetic tape reels in order to fully erase high-coercivity magnetic tapes, extensive heat buildup is produced in metallic reel materials (as distinguished from the magnetic tape materials themselves) so that the time during which the magnetic field may be applied is limited by the temperature resistance of the material used in the tapes. Another problem with prior tape erasers is that the tapes and their containers may be damaged during the erasing process if the tapes are picked up, moved about or forced into contact with the electromagnets due to the influence of the high magnetic field used for erasing. Such contact with the electromagnets may damage the tape containers or the electromagnets themselves. Another problem encountered is that residual noise may be left in the tapes after erasure, due to reduced strength of the erasing field at the center of the tape width. A further problem with prior magnetic tape erasers is that residual noise may be produced in the tapes which have undergone the erasure process, and where such noise is not directly related to the operation of the electromagnets used for erasing.